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Electrostatic Ignition Hazards in Industries
Dr.(Ms) Manju Mittal Sr. Principal Scientiest, Fire Research Laboratory, CSIR - CBRI Roorkee, India.
Static electricity is a frequent source of fire and explosion in industries. This paper presents an overview of electrostatic sparking phenomenon, evaluation of respective hazards in process industry and correlations for calculating their magnitude in practical engineering units, operations generating static electricity and measures to control this hazard.

Static electricity is a common source of ignition leading to fire and explosion in process industries. Electrostatic charge is developed due to relative motion between two dissimilar substances during various operations which may dissipate to earth or less charged objects resulting in a spark leading to fire, explosion, detonation and loss of plant, property and personnel. Various aspects for evaluation of a process plant for electrostatic hazard and providing adequate safety measures are outlined in this paper.

Electrostatic Phenomenon
Static charge generates from movement of electrons when two dissimilar substances come into close contact and then separate. Transfer of electrons occurs across the interface when two surfaces are in contact usually leading to line-ups of opposite electrical charges on two adjacent substances, at the boundary between them. Magnitude and polarity of charge being function of areas of contact surfaces, type of materials, surface temperature and separation velocity. Separation of two layers with opposite charges leads to potential difference. Electrical conductivity of material determines time required for charge to become neutralised and separation rate decides time available for discharge. Atmosphere is ionised if field strength exceeds a critical level, which may lead to discharge to some nearby earthed or less charged object as complete spark (responsible for majority of industrial fires and explosions) or partial breakdown- corona discharge depending on configuration of two surfaces acting as capacitor and field strength. Critical field strength values between large parallel electrodes in air at atmospheric pressure are given in Figure 1.

Evaluation of Fire and Explosion Hazard by Static Electricity
Fire and explosion hazard evaluation due to static electricity in industries and design of adequate safety measures require systematic analysis of type of material handled and their electrical properties; functional details of process, plant and machinery involved; and conditions which may lead to formation of flammable mixtures and electrostatic ignition. Properties required to assess fire/explosion hazard due to static electricity are: • Electrical conductivity of handled material.
• R elaxation time and time constant for relevant system.
• S trength of electric field.

Energy of spark discharged is given by
equation (1)
E = 1/2 CV2 = Q2 /2C

Where,
E - Stored energy (J)
C - Capacitance (F)
V - Electrostatic potential(V)
Q - Electrical charge (C)

Capacitance values of some plant items are given in Figure 2. Electrostatic ignition hazard exists when discharged energy is more than the minimum ignition energy of flammables (Figure 3).

A metallic system handling liquid hydrocarbons involves this type of hazard. Conductivity of liquids is due to presence of ionic impurities which yield charged particles on dissociation. Dielectric constant of liquid exerts a strong influence on dissociation degree of dissolved electrolytes.

Charge on fuel reaching the receiving tank and its capacity for producing incendiary discharge depend on charge generated in the system which varies with nature of impurities / additives in fluid, presence of filter, flow rate, residence time of fluid in system, electrical configuration of system, etc. Dissipation of electrostatic charge accumulated on liquid occurs according to equation (2)





Conduction properties of some hydrocarbons and water are given in Table 1. Safe liquid conductivities are

50x10-12S/m and 500x10-12S/m for pipe line velocities below and above 7 m/s. Voltages in static electricity are of the order of 10 kV and currents are 10-6 A to 10-5 A.

Controlling Electrostaic Hazards Control of Electrical Conductivity: Antistatic agents are added to increase conductivity and reduce relaxation times of some materials eg, a combination of a divalent or polyvalent metal salt of an acid such as dicarboxylic or sulphonic and a suitable electrolyte imparts a conductivity of ~10-8 S/m in a 0.1 per cent solution in benzene.

Conductivity of materials like fabrics, wood, paper is controlled by varying relative humidity of surrounding atmosphere. If it is maintained 60-70 per cent, charge leaks to ground. For warm surface eg, cloth passing over a heated roller and water repellent-oil surface, humidity is ineffective even if increased to 100 per cent.

Grounding, Earthing and Ionisation:
All plants, machinery and related equipment with a possible spark gap are bonded together to bring them to same potential to avoid electrical discharge. Objects should be earthed to dissipate charge to earth before it builds-up a high potential.

Ionisation: Ionisation of atmosphere surrounding the materials using an open flame, high voltage, sharply-pointed objects or radiation from a radioactive source, results in conduction of charge to earth through ionised air or its neutralisation.

Safe Operational Practices: Static electricity generation may be prevented by controlling the operations eg, reducing pumping speed, eliminating water or gas entrained by liquid before pumping, avoiding splash filling, inerting or mechanical ventilation to dilute the mixture below flammable range.

Industrial Operations Prone to Electrostatic Hazards Pumping, Flow, Loading and Storage of Organic Liquids: Electric field intensity generated during pumping of oil is proportional to square of pumping speed and flow velocity and addition of 6 per cent water may increase its value from ~ 5 kV/m to 100 kV/m. Reduction in pumping speed and use of larger diameter pipe is recommended for hydrocarbons with conductivities 10-12 S/m. Water presence in heavy oils, splash filling, high pumping rates and presence of filters may lead to fire and explosion problem due to static electricity during load switching in tank/tankers. To prevent hazard tanks must be properly earthed and purged by inert gas prior to filling. Hoses should be antistatic. Filling nozzles should be earthed and liquid velocity should be < 1 m/s.

Transfer of electrically charged liquid to a storage tank results in movement of unit charges with same polarity within liquid towards outer liquid surface. Splashing/ spraying of water by incoming stream, water distribution at the bottom and gas entrained by oil in pumping line, introduces electrostatic problems in storage tanks. In earthed metallic tanks, potential difference between any part of liquid surface and tanks becomes sufficiently high for a spark to jump to shell or earth which may lead to explosion. In a metal tank insulated from earth or one made of insulating material, spark discharge occurs between tank shell and earth. Explosion hazard may be eliminated by adding antistatic agents to liquids, using floating roof tanks or ionisation.

Steam or CO2 Extinguishers for Fire/ Explosion Safety: Steam jet at high pressure produces electrical charges in condensing steam cloud near nozzle end which may be acquired by an unearthed metallic object in vicinity of steam jets and lead to fire/explosion. Steam lines should be earthed and flow velocity kept low. Static electricity is generated during discharge of portable CO2 fire extinguishers which may be incendiary since solid particles, in high velocity streams of CO2, slide down the horn of an extinguisher. Potential of 40 kV, sufficient to cause a spark to jump across a 0.01 m gap may be produced on surface of horn of portable 6.8 kg CO2 extinguisher and ignite flammable mixture. These should never be used on tanks containing flammable mixtures.

Coating and Spreading Processes: Coating, impregnating and spreading for applying paint, lacquer, varnishes or rubber compounds to fabrics, paper or other materials using knives, rollers, spreaders, etc, produce static electricity during unrolling of material, passing it over rollers, or rolling on reels for coating or drying. Hazard can be controlled by bonding/earthing all metallic parts of equipment, providing electrostatic collectors and humidification.

Machinery/Vehicles Movement: Plastic film, paper and driving belts become charged on passing over earthed metal rollers since repetitive motion of material over a roller/ pulleys results in progressive build-up of charge and potentials of 108 V leading to hazardous situation in presence of flammables. Use of antistatic belts and earthed metal are advisable. Vehicles with dry pneumatic rubber tyres sometimes accumulate a charge due to rolling contact between tyres and road at high speed at the points of tyre separation from road.

Handling of Powders: Electrostatic problems occur in processing (mixing, grinding, screening or conveying) of non-conducting dusts. Dusts in bulk motion through a system may become charged and dust clouds cause a discharge to a less charged object or settled material, or carry charge to dust collecting vessel. Discharge between a charged cloud and settled material or an earthed metal object may easily have energy for ignition of flammable mixture. Charging of non-conducting particle flowing along the walls of pneumatic conveying lines leads to transfer of net charge into collecting vessel which may lead to explosion. Inerting, earthing, humidification or ionisation may be used as safety measures.

Human Being: People act as generators of static electricity and can acquire a charge. Conductive floors and footwear are used to provide direct leakage path for charge. Persons working in hazardous locations should wear clothing woven from synthetic fibers, made conductive by interwoven carbon filaments. An average person has capacitance between 200-300 pF and can become charged to 10-30 kV when isolated from ground and spark energy can be 10- 135 mJ.

Conclusion The information presented in this paper will prove helpful in evaluating electrostatic hazards in process industries. Electrostatic phenomena are encountered so widely that it is not possible to cover all cases and research in this field continues providing new information. Plant operators must be made aware of ways by which electrostatic hazards may arise in their plant operations and trained to evaluate to what extent design, operating procedure, maintenance and inspection ensure hazards prevention/ mitigation.